The presence of substantial amounts of hydrogen sulfide in various streams, especially those arising from chemical and petrochemical plants and feedstocks, has required the development of suitable processes for its removal because hydrogen sulfide frequently is an undesirable contaminant for diverse reasons. The Claus process enjoys widespread usage in converting hydrogen sulfide to sulfur but suffers from inherent limitations, two of which are especially significant in the context of this application. One limitation results from its conversion efficiency in the range of 93-97%, as a consequence of which tailgas emissions still contain unacceptably high hydrogen sulfide levels, especially in view of air quality standards and goals. Another limitation results from economy of size which makes the Claus process commercially feasible only where large amounts of hydrogen sulfide are to be removed. The Claus process simply is not feasible for small streams, or for streams containing relatively low (not more than about 1,000 ppm) levels of hydrogen sulfide.
Several other processes have been developed for hydrogen sulfide removal from streams with lower levels of hydrogen sulfide, with those which have achieved some measure of commercial success being oxidative processes. See D. A. Dalrymple, T. W. Trofe, and J. M. Evans, Chemical Engineering Progress, March, 1989, pp. 43-49. The best known of these is the Stretford process which converts hydrogen sulfide to sulfur in a vanadium-based oxidation process using an oxygen transfer agent such as anthraquinone disulfonic acid to catalyze the oxidative regeneration of vanadium-(V) from vanadium-(IV). Like the Claus process, the Stretford process also has some inherent disadvantageous characteristics which severely limit its usefulness. Some of the hydrogen sulfide is converted to thiosulfate and sulfate salts rather than elemental sulfur. Discharge of these salts often is environmentally unacceptable, and the presence of significant vanadium levels merely exacerbates the problem; Dalrymple et al. have estimated disposal costs at $130-260 per kiloliter. The Stretford process is most easily used for gaseous streams and is not readily adaptable to liquid streams. In liquid phase streams the hydrogen sulfide usually is extracted with caustic solution with subsequent disposal of the caustic sulfide. With increasingly stringent environmental regulations caustic sulfide disposal is also becoming increasingly expensive and environmentally undesirable.
The hydrotreating of various petrochemical feedstocks generally leads to formation of hydrogen sulfide at levels necessitating its removal. As the amount of hydrotreated streams increases, as the environmental susceptibility to sulfates and thiosulfates increases, and as the demand for lower levels of hydrogen sulfide in various streams, especially petrochemical feedstocks, increases there is a more insistent and persistent demand for hydrogen sulfide removal from streams, both gaseous and liquid, in an environmentally benign manner. What seems particularly desirable is a non-oxidative method of hydrogen sulfide removal adaptable to a broad variety of hydrogen sulfide-containing streams.
Our invention is a non-oxidative method of hydrogen sulfide removal from gaseous or liquid streams, whether aqueous or non-aqueous, which converts hydrogen sulfide to mercaptan with subsequent removal of the mercaptan by art-recognized and environmentally neutral methods. The best example of the latter is the mild oxidation of mercaptan to disulfides which may remain in the stream as an innocuous contaminant or be removed from the original H.sub.2 S-containing stream by known methods. See R. A. Meyers, Handbook of Petroleum Refining Processes, McGraw-Hill Book Company, (1986), part 9. More recently we have developed a method of mercaptan removal which employs the addition of mercaptans to olefins to form thioethers, which also are generally innocuous contaminants; see U.S. Pat. No. 4,775,462. The invention within is somewhat related to the latter.
In greater particularity, our invention employs the reaction of hydrogen sulfide in hydrogen sulfide-containing streams with olefins to form mercaptans and, to a minor extent, thioethers. The formed mercaptans are converted to disulfides in an art-recognized and environmentally sound manner. The removal of hydrogen sulfide is efficient and applicable to both gas and liquid streams. Since prior art methods generally are not feasible for hydrogen sulfide removal from liquid streams, our invention fills a burgeoning industrial need. But since our process is adaptable to both gaseous and liquid feeds its versatility is economically beneficial. The method which is our invention is selective with respect to hydrogen sulfide removal, employs mild conditions where even relatively sensitive components remain unaffected, and routinely can attain residual hydrogen sulfide levels of 5 ppm or less.